1. Field of the Invention
This invention relates to improvements in a process for polymerizing a styrene monomer or a styrene monomer having a rubber-like polymer dissolved therein by using an organic peroxide as the catalyst.
2. Description of the Prior Art
In the prior art, there are a number of well-known processes for the production of styrene polymers on an industrial scale. For example, styrene polymers can be produced simply by heating a styrene monomer or a styrene monomer having a rubber-like polymer dissolved therein. Alternatively, they can also be produced by adding an organic peroxide (for example, benzoyl peroxide) to the aforesaid monomer and subjecting the resulting reaction mixture to polymerization.
According to the recent trend of the market, high-molecular-weight styrene polymers are preferred with a view to improving the mechanical and thermal strength of styrene resins. One means of achieving such improved resin performance is to increase the average molecular weight of styrene polymers. This can be realized either by reducing the polymerization temperature or by decreasing the amount of polymerization initiator added. However, since there is a conflicting relationship between the polymerization rate and the average molecular weight of the resulting polymer, it is generally difficult to produce styrene polymers having a high average molecular weight at a high polymerization rate, that is, with high productivity.
In order to solve these problems, a number of processes have been proposed. More specifically, Japanese Patent Publication No. 797/1977 discloses a process which comprises effecting bulk or solution polymerization by using 2,5-dimethyl-2,5-di(tert-butylperoxy)hexyne-3 as the catalyst; Japanese Patent Publication No. 42834/1977 discloses a process which comprises effecting continuous polymerization by using 2,5-dimethyl-2,5-di(benzoylperoxy)hexane, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane or the like as the catalyst and keeping the contents of the polymerization zone in such a mixed state as to make them substantially homogeneous; and Japanes Patent Laid-Open No. 107994/1979 discloses a process which comprises effecting suspension or bulk-suspension polymerization by using 1,1-bis(tert-butylperoxy)cyclohexane or 1,1-bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane as the catalyst.
These prior art processes are based on the technological conception that, in order to improve the mechanical and thermal strength of styrene resins, efforts should be concentrated solely on the preparation of high-molecular-weight styrene polymers in the polymerization step. In practice, however, the high molecular weight of the styrene resins which have been formed into molded articles are important in achieving the desired mechanical and thermal strength. In conventional processes for the production of styrene polymers, the styrene polymer prepared in the polymerization step is subjected to thermal history during the course of after-treatments such as the removal of volatiles and the incorporation of additives, and further heated in the molding step. Thus, the fact is that its average molecular weight is considerably reduced in consequence. Accordingly, in order to produce molded articles having a high average molecular weight, it is most desirable from an industrial point of view to not only achieve a high average molecular weight in the polymerization step but also minimize the reduction in average molecular weight during the course extending from the completion of the polymerization to the fabrication of molded articles.